In the field of medical imaging such as PET (positron emission tomography), MRI (magnetic resonance imaging), and ultrasonic imaging, many clinical researches have been made on molecular imaging using nanoparticle contrast agents (see, for example, M. F. Kircher and J. K. Willmann, “Molecular Body Imaging: MR Imaging, CT, and US. Part I. Principles”, Radiology, Vol. 263, pp. 633-643, 2012). In addition, clinical researches have been made on molecular imaging using nanoparticles and the like including gold as a heavy metal in PCCT (photon counting CT) called next-generation CT (computed tomography) (see, for example, M. Shilo, et al., “Nanoparticles as computed tomography contrast agents: current status and future perspectives”, Nanomedicine, Vol. 7, pp. 257-269, 2012).
The EPR (enhanced permeability and retention) effect occurs in neighboring blood vessels and new nutrient vessels for cancer cells which have progressed to a certain degree. The EPR effect is a phenomenon in which the enhancement of vascular permeability due to the expansion of the gaps between vascular endothelial cells occurs together with the enhancement of the retention of vascular permeability substances due to the undevelopment of a lymphoid system. It is known that vascular endothelial cell gaps are about 5 nm to 50 nm in a normal state, whereas vascular endothelial cell gaps are about 150 nm or more under the EPR effect. In molecular imaging using nanoparticles, imaging is basically performed by using nanoparticles having a single particle size even with slight variations. For this reason, if the particle size is smaller than a vascular endothelial cell gap, it is difficult to image the blood vessel itself, even though it is possible to image the stromal system of a cancer tissue. In contrast to this, if the particle size is larger than a vascular endothelial cell gap, it is difficult to image the stromal system of a cancer tissue, even though it is possible to image the blood vessel itself.